I have sent a calendar invite earlier to a group and recently sent an update. Some of the names appeared under "optional" instead of "to" even though i didn't amend the list . Can you please advise how it happened and how we can prevent it? Im on Outlook 2010 and my organisation is using Ms Exchange 2010 server .

Hi G.Thiru,

This is an expected result.

When Outlook receives a meeting acceptance, the sender(user@domain.com) of the acceptance is compared to the recipients(Group@domain.com) in the meeting attendee list. If that recipient is not listed(FALSE in case of Group\DL), it is assumed to be "optional" and added to the list of attendees.

Read below for details.

Outlook Meeting Request sent to DL Shows Members As Optional:

https://support.microsoft.com/en-us/kb/214633#/en-us/kb/214633